parallel feeders 2

[tito103]

Hello,
I was infrared scanning a 2000 amp service 480 volts. The cables ran from a 2000 amp transfer switch too a 2000 amp main circuit breaker. The feeders are in cable and installed in cable tray. Approximate cable length 80ft. There where 5 parallel 600mcm feeders per phase.The problem I was having is a couple of the feeders are drawing twice the current then the other feeders in the same phase? Does this mean that the cables drawing less current have a higher contact resistance?

 

[davidbeach]

PWR, you're right, the current in a segment of the circuit would divide inversely proportionally to the relationship of the impedances of that segment. I was mentally comparing the conductor impedances with the rest of the system impedance, but that was not correct. Sometime when I've nothing better to do (not any time soon unfortunately) I'll try to remember to do some modeling on this, I'm intrigued that you could get that much difference to have over three times the impedance on one conductor as on another.

None of that changes the fact that I don't like (can't make sense of) tito103's numbers; if they had a whole lot more spread to them they'd make much better sense.

 

[waross]

Hello davidbeach
You seem to have missed my post of 16 Mar 06 8:05, or more likely we were posting at the same time.

I am wondering if this installation may be;
One bundle A-A-A-A-B, one bundle B-B-B-B-C, and one bundle C-C-C-C-A.
Comments?/

I think this arrangement may explain the unusual division of current.
tito103, Can you check and see if the cables are in fact arranged similar to this?
Thanks.
respectfully

 

[davidbeach]

waross, I saw that post, but read it as laid out flat in the bottom of the tray. Without doing any modeling, it seems to me that your configuration (laid out flat in a tray) would provide at least three different currents, and probably a spread more like what PWR reported on his measurements. If you mean bundle as with tie wrap around each bundle and wide separations (wide as compared to cable diameter) between bundles, maybe, but best to hope that the high current cable is on the top of the bundle or there will be serious heating problems.

What we really need now is for tito103 to respond with much more information than he (she? - just can't tell with most handles) has provided to date.

 

[tito103]

hey guys, the cables are placed like this.
cccccbbbbbaaaaannnnn the width of the tray. None of the cables are stacked on top of each other, and they are all tie-wraped to the tray individually.

 

[waross]

Thanks tito103.
I agree with davidbeach and PWR. Those readings don't make sense. The high current is to be expected. The low currents being equal is not normal. When you say "the other four cables had approx 200 amps.", do you mean within 5%, 10% 20%?
I am thinking about an ammeter I had once that misbehaved badly when exposed to the magnetic fields of surrounding conductors.
I had another ammeter that had a habit of sticking mid scale when static built up on the face. That made a lot of readings look equal that actually weren't. It wasn't particularly accurate either.
Is it possible that one set of 600 MCM cables is really 750 MCM?
I would not normally make such suggestions but these readings do not seem to be a normal.
respectfully

 

[davidbeach]

tito103, you should have five different current measurements per phase based on that configuration. What about the currents in the other two phases and neutral? Have you physically inspected the entire run and found that nothing comes between the conductors at any point; any metallic clamps, any all thread hanger rods, anything?

 

[KJvR]

tito103,

My first thought was also due to the unsimmetric cable lay-out, but your currents would have varied as already stated. My second thought was that your cables have screens and maybe one of the screen connections was bad, causing additional unbalance. Although you can get such high variances in current, it still does not explain why it happens on all three phases and the other cables' currents are more or less the same at 200 A.

I would like to support the idea of a different set of cables (raised by warros). The installation is already 35 years old and expansions surely must have happened. An additional cable could have been installed and it might be that a Cu conductor in stead of an Al conductor was installed or just a different size cable.

KJvR

 

[tito103]

Hey guys great info! as far as the cable placement, there is only one row of cables. None of the cables are placed on top of each other. They are straight across the cable tray. I checked all the cables and they are all copper 600mcm. As far as ferrous clips or supports there are none. The only supports are plastic tie wraps. During an infrared inspection I picked up on this problem. The first thing I did was amprobe the conductor with the most heat, that was the cable with 560 amps. The cable terminated next to it was drawing about 200 amps and looked normal during the infrared inspection. I could not take current readings from all the cables because it was to dangerous. Hope this info is a little more helpful.

 

[waross]

Thanks for the clarification tito103
As you realised we were under the impression that the currents had been measured in each cable. The situation is not good but is to be expected with that cable arrangement.
The Canadian Electric Code recomends ABCABCABCABCABC for parallel feeders laid flat in a cable tray. What says the NEC fellows?
You have a good case for some remedial work reconnecting the cables in an approved or recommended configuration on the next scheduled shutdown.
respectfully

 

[matwong]

What I suspect is the connection problem at your terminals of transfer switch or MCCB. I presume the cable carries the highest current is connected directly to the terminals of the equipment (and the same applied to other phases), whereas the other cables are either cascaded or connected to the undersized extended terminal. As itsmoked commented, some pictures might be useful.